In the last years, several efforts have been made for proposing new optical communication modulation formats. The common goal for those new formats is to overcome the limitations imposed by the traditional modulation technique (i.e. the Intensity Modulated with Direct Detection systems, IM-DD) in terms of bandwidth requirements for both the optical and electrical components.
The Wavelength Division Multiplexing (WDM) technique, where several digital optical signals at different wavelengths are transmitted together in the same optical fiber, is now commonly used to increase the overall transport capacity [bit/s]; this makes of the spectral efficiency [bit/s/Hz], defined as the ratio between the bit rate R [bit/s] for each WDM channel and the frequency spacing among these, a parameter of great importance in the design of optical transport networks. IM-DD systems have typical spectral efficiencies of 0.4 bit/s/Hz, and using complex techniques like the Polarization Interleaving (PI), Polarization Domain Multiplexing (PDM) and Vestigial Side Band (VSB) it is hardly possible to reach the value of 0.8 bit/s/Hz.
Next generation WDM channels are planned to transmit at bit rates of R=40, 80 and 160 Gbit/s; the feasibility of commercial IM-DD transmitters at those bit rates is not obvious, because the required bandwidth for the electronics and opto-electronics is comparable to R. The development of stable 40 GHz electronics has emerged in the last few years, and is still characterized by high production costs, while the development of electronics with cut-off frequency approaching to 80 or 160 GHz is still far to come.
Modulation formats alternative to standard IM-DD may help to increase the system spectral efficiency, to reduce the bandwidth requirements both in the optical and electrical domain, and/or to improve the transmission performances and tolerances to the linear (Group Velocity Dispersion, Polarization Dispersion) and nonlinear (Kerr and Raman effects) optical impairments. Recently, optical systems using polibinary optical signals or with multilevel amplitude have been proposed [S. Walklin et al. “Multilevel signaling for increasing the reach of 10 Gb/s lightwave systems”, IEEE Journal of Lightwave Technology 17, pp. 2235-2247 (November 1999)]. Multilevel optical phase has been described in [R. A. Griffin et al., “10 Gb/s optical differential quadrature phase shift key (DQPSK) transmission using GaAs/AlGaAs integration”, proc. of OFC 2002, FD6-1]. The use of optical phase modulation with pulsed optical carrier (Return-to-Zero Differential Phase Shift Keying, RZ-DPSK) has been introduced in [T. Miyano et al., “Suppression of degradation induced by SPM/XPM+GVD in WDM transmission using bit-synchronous intensity modulated DPSK”, proc. of OECC'00, vol. 14D3 (2000)]. The combined use of Non-Return-to-Zero (NRZ) intensity modulation and phase modulation has been proposed by [M. Ohm et al. “Quaternary optical ASK-DPSK and receivers with direct detection”, IEEE Photonics Technology Letters 15, pp. 159-161 (January 2003)], although this method requires the use of a reduced extinction ratio for the intensity. The combined use of dark pulse intensity modulation and phase modulation has been proposed in February 2002 and January 2003 by the inventor [Patent Application Publication no. US2003/0147646 A1, IT-RM2002A000056]; the proposed format has the advantage that both the optical intensity and phase have maximum extinction ratio and performances.
The optical formats proposed above require reduced bandwidth for the electronics and the opto-electronics. Conversely, the use of multilevel amplitude or phase, as well as the use of intensity modulation with reduced extinction ratio, has the disadvantage to lower the received eye opening, causing the reduction of the system performances. Besides this, all the proposed formats require complex transmitter schemes, with heavy electronics or with at least one optical modulator for each tributary multiplexed into a WDM channel.